[GitHub] [arrow-datafusion] Dandandan commented on a change in pull request #403: add `first_value`, `last_value`, and `nth_value` built-in window functions

[GitBox <gi...@apache.org>]

Dandandan commented on a change in pull request #403:
URL: https://github.com/apache/arrow-datafusion/pull/403#discussion_r641480304



##########
File path: datafusion/src/physical_plan/expressions/nth_value.rs
##########
@@ -0,0 +1,223 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Defines physical expressions that can evaluated at runtime during query execution
+
+use crate::error::{DataFusionError, Result};
+use crate::physical_plan::{
+    window_functions::BuiltInWindowFunctionExpr, PhysicalExpr, WindowAccumulator,
+};
+use crate::scalar::ScalarValue;
+use arrow::datatypes::{DataType, Field};
+use std::any::Any;
+use std::convert::TryFrom;
+use std::sync::Arc;
+
+/// first_value expression
+#[derive(Debug)]
+pub struct FirstValue {
+    name: String,
+    data_type: DataType,
+    expr: Arc<dyn PhysicalExpr>,
+}
+
+impl FirstValue {
+    /// Create a new FIRST_VALUE window aggregate function
+    pub fn new(expr: Arc<dyn PhysicalExpr>, name: String, data_type: DataType) -> Self {
+        Self {
+            name,
+            data_type,
+            expr,
+        }
+    }
+}
+
+impl BuiltInWindowFunctionExpr for FirstValue {
+    /// Return a reference to Any that can be used for downcasting
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn field(&self) -> Result<Field> {
+        let nullable = true;
+        Ok(Field::new(&self.name, self.data_type.clone(), nullable))
+    }
+
+    fn expressions(&self) -> Vec<Arc<dyn PhysicalExpr>> {
+        vec![self.expr.clone()]
+    }
+
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn create_accumulator(&self) -> Result<Box<dyn WindowAccumulator>> {
+        Ok(Box::new(NthValueAccumulator::try_new(
+            1,
+            self.data_type.clone(),
+        )?))
+    }
+}
+
+// sql values start with 1, so we can use 0 to indicate the special last value behavior
+const SPECIAL_SIZE_VALUE_FOR_LAST: u32 = 0;
+
+/// last_value expression
+#[derive(Debug)]
+pub struct LastValue {
+    name: String,
+    data_type: DataType,
+    expr: Arc<dyn PhysicalExpr>,
+}
+
+impl LastValue {
+    /// Create a new FIRST_VALUE window aggregate function
+    pub fn new(expr: Arc<dyn PhysicalExpr>, name: String, data_type: DataType) -> Self {
+        Self {
+            name,
+            data_type,
+            expr,
+        }
+    }
+}
+
+impl BuiltInWindowFunctionExpr for LastValue {
+    /// Return a reference to Any that can be used for downcasting
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn field(&self) -> Result<Field> {
+        let nullable = true;
+        Ok(Field::new(&self.name, self.data_type.clone(), nullable))
+    }
+
+    fn expressions(&self) -> Vec<Arc<dyn PhysicalExpr>> {
+        vec![self.expr.clone()]
+    }
+
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn create_accumulator(&self) -> Result<Box<dyn WindowAccumulator>> {
+        Ok(Box::new(NthValueAccumulator::try_new(
+            SPECIAL_SIZE_VALUE_FOR_LAST,
+            self.data_type.clone(),
+        )?))
+    }
+}
+
+/// nth_value expression
+#[derive(Debug)]
+pub struct NthValue {
+    name: String,
+    n: u32,
+    data_type: DataType,
+    expr: Arc<dyn PhysicalExpr>,
+}
+
+impl NthValue {
+    /// Create a new NTH_VALUE window aggregate function
+    pub fn try_new(
+        expr: Arc<dyn PhysicalExpr>,
+        name: String,
+        n: u32,
+        data_type: DataType,
+    ) -> Result<Self> {
+        if n == SPECIAL_SIZE_VALUE_FOR_LAST {
+            Err(DataFusionError::Execution(
+                "nth_value expect n to be > 0".to_owned(),
+            ))
+        } else {
+            Ok(Self {
+                name,
+                n,
+                data_type,
+                expr,
+            })
+        }
+    }
+}
+
+impl BuiltInWindowFunctionExpr for NthValue {
+    /// Return a reference to Any that can be used for downcasting
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn field(&self) -> Result<Field> {
+        let nullable = true;
+        Ok(Field::new(&self.name, self.data_type.clone(), nullable))
+    }
+
+    fn expressions(&self) -> Vec<Arc<dyn PhysicalExpr>> {
+        vec![self.expr.clone()]
+    }
+
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn create_accumulator(&self) -> Result<Box<dyn WindowAccumulator>> {
+        Ok(Box::new(NthValueAccumulator::try_new(
+            self.n,
+            self.data_type.clone(),
+        )?))
+    }
+}
+
+#[derive(Debug)]
+struct NthValueAccumulator {
+    // n the target nth_value, however we'll reuse it for last_value acc, so when n == 0 it specifically

Review comment:
       Yeah I agree that sounds like a more idiomatic way.




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